Chlorine is commonly used to disinfect sewage treatment plant process streams and for biofouling control in cooling systems. Because chlorine is such a highly effective disinfecting/oxidizing agent, any chlorine unused in the process is just as effective in destroying aquatic life.
In order to eliminate or substantially remove the residual chlorine from a process stream, a dechlorination agent or device is often used. Typically, sulfur dioxide (SO.sub.2) is added to the process stream to react quantitatively with the chlorine residual. If SO.sub.2 is added in excess of the amount of chlorine residual in the process stream, the chlorine will be completely eliminated.
The amount of unreacted dissolved SO.sub.2 (i.e., dechlorination residual) remaining in the process stream is preferably maintained at low, but positive, concentration to insure chlorine residual removal and minimize oxygen consumed. This sort of treatment not only protects aquatic life, but results in more efficient use of the C1.sub.2 and SO.sub.2 reagents added to the process stream. However, there are no completely acceptable methods or systems available for directly monitoring and controlling the amount of dechlorination residual in a continuous process stream to which a chlorine disinfectant/oxidant has been previously added.
All of the prior systems are expensive to install and have substantial operational costs associated with maintenance, sample pumping, and reagent costs. These systems are often inherently unstable, producing measurement errors that typically exceed safe regulatory residual limits by several orders of magnitude. This is especially undesirable since dechlorination control continues to be the subject of more stringent government environmental regulations.
Clearly what is needed is a method and system for directly measuring and controlling a continuous process stream for dechlorination residual. The method and system should permit a chlorine residual to be completely eliminated within the process stream while the amount of dechlorination residual is minimized. Monitoring and control should be conducted under conditions where relatively small volumes of the reagent and process sample are used. Moreover, the reagents used should be relatively stable and should not be subject to chemical reaction interferences with other contaminants which may be present within the process stream. The present invention fills that need.